2.1: Intermolecular Forces

Three intermolecular forces that play a huge role in determining physical properties are van der Waals forces, dipole-dipole interactions, and hydrogen bonding. Of the three, van der Waals forces are the weakest and hydrogen bonds are the strongest.

Van der Waals Forces

Van der Waals forces, also known as London dispersion forces, are momentary attractions felt between molecules because of temporary dipoles. These dipoles are created when electrons become concentrated in one part of a molecule. This happens due to the random motion of electrons. Although all electrons have the same charge and repel each other, it is very rare for electrons to be distributed evenly around the nucleus because the electrons are constantly in motion. Most of the time, there will be different concentrations of electrons throughout the molecule. When this happens, part of the molecule with have a greater density of negatively-charged electrons, creating a negative pole. At the same time, a positive pole forms in the area of the molecule that lacks electrons. This temporary separation of charges is called an induced dipole. The charged ends of the induced dipole will be attracted to oppositely-charged ends of other dipoles (either permanent or induced), creating a momentary bond that holds the molecules together. The general rule is that the larger the molecule, the greater its van der Waals interaction with other molecules. This is because larger molecules have a greater surface area where van der Waals interaction can take place.

Dipole-Dipole Interactions

A molecule is considered polar when two bounded elements in the molecule have a large difference in their electronegativities. When this happens, the more electronegative atom will “hog” the electrons in the bond. This increases the electron concentration around the more electronegative atom, causing the atom to obtain a slight negative charge. The other atom that has had its electrons taken away will have a slight positive charge. This separation of charges is called a dipole. Similar to van der Waals forces, the charged ends of the dipole are attracted to oppositely-charged dipoles on other molecules. The interactions between dipoles are referred to as dipole-dipole interactions. Compared to the induced dipoles involved in attractive van der Waals forces, these dipoles are permanent. As a result, dipole-dipole interactions tend to be stronger than van der Waals forces.

Hydrogen Bonding

Hydrogen bonding is a phenomenon exhibited by water molecules, carboxylic acid molecules, and other molecules that contain a hydrogen atom and an exceptionally electronegative element, such as nitrogen, oxygen, or fluorine. Let’s look at water molecules. Water molecules are polar; the oxygen atom has a negative charge and the hydrogen atoms have positive charges. The negatively-charged oxygen atom in one water molecule is attracted to the positively-charged hydrogen atoms in another water molecule. At low temperatures, this attraction is strong enough to form a loose bond called a hydrogen bond. This increases the boiling point of the substance because this hydrogen bond has to be broken before the substance can evaporate.